Generally, an adhesive sheet obtained by applying a pressure-sensitive adhesive agent to a substrate has been used in various fields because an adhesive sheet can easily adhere to an adherend by being pressed. In recent year, plastic molding has been widely used for many reasons, such as reducing the weight of products. As a result, the use of an adhesive sheet which adheres to a plastic molding has increased. Examples of a plastic molding include a product obtained by molding a resin such as polycarbonate, polymethyl methacrylate, polystyrene or an ABS resin. When an adhesive sheet (for example, an adhesive label) including a substrate which has a gas-barrier property was adhered to a surface of these plastic moldings for the purpose of decoration and the like, the gas is generated from the plastic molding and the bubbles are formed between the adhesive sheet and plastic molding. As a result, swelling or floating, that is a blister, is likely to be generated. When such a blister occurs, the appearance of a label is impaired. As a result, the decorative property of the adhesive sheet decreases remarkably.
In order to solve this problem, for example, Patent Document 1 discloses an acrylic adhesive composition in which the copolymer obtained by radical polymerization of methacrylic acid ester and a compound which has a carboxyl group capable of a copolymerization and a tertiary amine which has a vinyl group is included.
In addition, Patent Document 2 proposes an adhesive sheet having a blister resistance in which a copolymer consisting of an acrylic acid ester including alkyl group having 4 to 12 carbon atoms and 0.1 to 10 wt % of a polar monomer such as acrylic acid or 2-hydroxyethyl acrylate and an aziridine-based cross-linking agent are added.
Further, Patent Document 3 discloses an adhesive sheet including an adhesive component and a hardening component which is an acrylic monomer or oligomer as a constituent of an adhesive layer. However, the hardening component causes a reduction of the flocculation capacity of the adhesive layer. Further, when the compatibility with the adhesive component is not good, the adhesive layer might become cloudy.
Patent Document 4 discloses an adhesive composition in which a resin composition (1) having a specific molecular weight which was obtained by copolymerization of a methacrylic acid alkyl ester having 1 to 12 carbon atoms with a unsaturated monomer including carboxyl group and one or two or more monomer selected from a methacrylic acid alkyl ester having 1 to 20 carbon atoms, a methacrylic acid cycloalkyl ester, a benzyl-methacrylic acid or styrene and a resin composition having a specific Tg and molecular weight which was obtained by copolymerization of a unsaturated monomer including an amino group are added.
Further, Patent Document 5 discloses an adhesive composition having a blister resistance in which a copolymer having a specific molecular weight which was obtained by copolymerization of an unsaturated monomer including carboxyl group with an unsaturated monomer including amino group by as a main component of an acrylic acid ester including alkyl group having 1 to 12 carbon atoms, and a cross linking agent having glycidyl group are added.
Patent Document 6 discloses a resin composition having a molecular weight of 1,500,000 or more and the weight average molecular weight/the number average molecular weight of 4.0 or less.